This invention relates to a method of recovering energy in a forming section of a papermaking or boardmaking machine, wherein stock from a headbox is fed into a forming zone of a forming section, the forming zone including at least one looped forming fabric curving along a convex surface of a support member, and water is drained from the stock through the forming fabric in the forming zone to form a paper or board web, the water passing through the fabric and being thrown out from the forming zone with a substantial velocity. Moreover, the invention relates to an apparatus for forming a web in a papermaking or boardmaking machine.
Paper and board is today produced at very high speeds, and especially tissue paper, newsprint, and magazine paper. For tissue the machine speed has today reached 2000 m/min. When forming the fiber web, for instance in a double wire former, stock is injected by the headbox in between two forming clothings, which both run over a wire support, such as a forming roll. The outer clothing is a wire, which is permeable to water. The other clothing (e.g. a felt or a wire) is intended to carry the web for further processing. The stock has a fiber concentration of between 0.1 to 0.5% and the flow is about 0.5 m3/sec per meter of web width in the cross-machine direction. The forming of the web occurs by means of the water within the stock being drained through the outer flexible fabric, i.e. the wire, such that only a minor portion of the water is carried on by the fiber web. The water is squeezed out by the static pressure which is applied by means of the wire which is pretensioned by lead rolls against the forming roll. Due to the above mentioned force the water that leaves through the wire will theoretically normally have a larger speed than the peripheral speed of the forming roll. Since enormous amounts of water are drained, e.g. in a normal large tissue machine (6 meters wide) the flow of drainage water is about 3 m3/sec, it is realized that large amounts of energy are released at this point of a paper machine. Hitherto, none of this energy has been recovered, at least not the kinetic part thereof, the water merely being collected in a white water tray for recirculation. The same problem is relevant also in connection with single wire formers using a single wire and a forming roll or in a blade former type of a forming section, wherein a forming roll is not required.
It is an object of the invention to minimize the above-mentioned disadvantages by providing a method and an apparatus for recovery of a substantial part of the kinetic energy of the drained white water in a paper machine. In accordance with the invention, a method for operating a forming section of a paper machine comprises the steps of discharging stock from a headbox into a forming zone of the forming section, the forming zone including at least one forming fabric arranged in a loop and traveling in the forming zone along a curvilinear path, water from the stock being drained through the at least one forming fabric in the forming zone such that the water exits with a substantial velocity from the at least one forming fabric; and capturing the water exiting from the at least one forming fabric and converting kinetic energy of the water into a useful form for supplying power to a further device in the papermaking machine.
Preferably, the kinetic energy of the water is converted into useful form by a movable component placed in the path of the water exiting from the forming zone such that the moving water causes the movable component to be moved. In preferred embodiments of the invention, the movable component is a turbine that is rotated by the moving water. The turbine preferably is a reaction turbine, more preferably a cross-flow type of reaction turbine such as a Banki turbine. A rotating shaft of the turbine can be used for supplying mechanical power to a further device such as a pump, or can be used for operating an electrical generator, which in turn can supply electrical power to a further device. It is particularly preferred to use the energy provided by the turbine to power a stock pump of the paper machine.
In preferred embodiments of the invention, the at least one forming fabric in the forming zone passes over a convex surface of a support member, and the turbine and the support member are disposed on opposite sides of the forming fabric. Preferably, at least one guide plate is disposed adjacent the forming fabric for guiding water expelled therefrom into the turbine. The convex surface preferably has a substantially constant radius of curvature in the forming zone and the at least one guide plate comprises a first guide plate a major portion of which is spaced radially outward from and generally parallel to the convex surface in the forming zone. The major portion of the first guide plate preferably also has a substantially constant radius of curvature, which advantageously is between about 100 percent and 120 percent of the radius of curvature of the convex surface of the support member. The support member can be of various types depending on the type of former used in the paper machine. For example, the support member can be a forming roll, a forming shoe, or a series of dewatering blades. Where a forming roll is employed, the first guide plate preferably has an angular extent of about 20xc2x0 to 90xc2x0 about a center of the forming roll. The optimal angular extent of the guide plate can depend on the type of forming roll used. Advantageously, when the forming roll has an impermeable surface the first guide plate has an angular extent of about 40xc2x0 to 80xc2x0, whereas when the forming roll comprises a vacuum forming roll the first guide plate has an angular extent of about 20xc2x0 to 50xc2x0.
By the invention surprisingly large amounts of energy may be recovered from the kinetic energy of the water which is taken out from the stock during the dewatering process in connection with the forming of the web. Calculations show that for a tissue twin wire machine having a 6-meter wide headbox and a machine speed of 1800 m/min up to 800 kW can be recovered, which implies a saving of about 2 million SEK/year. Since the investment cost is relatively moderate, the pay-off time can be made very short depending on the price of electricity.